Marine trunk piston engines generally use Heavy Fuel Oil (‘HFO’) for offshore running. Heavy Fuel Oil is the heaviest fraction of petroleum distillate and comprises a complex mixture of molecules including up to 15% of asphaltenes, defined as the fraction of petroleum distillate that is insoluble in an excess of aliphatic hydrocarbon (e.g. heptane) but which is soluble in aromatic solvents (e.g. toluene). Asphaltenes can enter the engine lubricant as contaminants either via the cylinder or the fuel pumps and injectors, and asphaltene precipitation can then occur, manifested in ‘black paint’ or ‘black sludge’ in the engine. The presence of such carbonaceous deposits on a piston surface can act as an insulating layer which can result in the formation of cracks that then propagate through the piston. If a crack travels through the piston, hot combustion gases can enter the crankcase, possibly resulting in a crankcase explosion.
It is therefore highly desirable that trunk piston engine oils (‘TPEO’s) prevent or inhibit asphaltene precipitation. The prior art describes ways of doing this.
WO 2010/115594 (“594”) and WO 2010/115595 (“595”) describe the use, in trunk piston marine engine lubricating oil compositions that contain 50 mass % or more of a Group II basestock, of respective minor amounts of a calcium salicylate detergent and of a polyalkenyl-substituted carboxylic and anhydride. The data in “594” and “595” show that the combination gives rise to improved asphaltene dispersancy.
“594” and “595” are not, however, concerned with the economics of treating TPEO's to inhibit ‘black paint’ formation when a Group I base oil is used. A considerable cost arises from the amount of detergent soap that is used, i.e. the detergent other than the basic material. It is now found that, by using relatively small additions of the above anhydride, it is possible to achieve good asphaltene dispersancy properties at lower, and therefore more economic levels, of soap.